The present invention relates to systems for continuously manufacturing composite, adhesively bonded products in which pressure and microwave heat are applied simultaneously to curable assemblies. The adhesive bonding agent is thereby cured or set while the product is pressed and/or maintained at the desired dimensions and density. The invention further relates to microwave methods for curing resins used as binders or adhesives for materials, such as wood particles, wood chips, wood wafers, wood strips, wood fibers and wood veneers, used in the production of chip board, hard board, particle board, wafer board, plywood and other composite products.
Wood products of this type have been subjected in the past to heat and pressure in hot presses. Wood, however, is a relatively poor conductor of heat, and the heat from the platens of the hot press can only be directed against the outer surfaces of the wood product being formed. Consequently, considerable time was required for the necessary heat to penetrate to the center of the wood product and to cure the resin therein. If the temperature was increased beyond a certain amount to reduce the curing time required, scorching or charring of the outer surfaces of the wood product resulted. These higher temperatures also were difficult and expensive to attain since they required greater steam pressure and additional equipment. Additionally, at higher temperatures, water which may be entrapped can result in steam explosion in the product.
Numerous attempts have been made to use radio frequency (R.F.) energy, that is, dielectric heating, to cure the resin. Where R.F. heating techniques were used and especially where the phenolic resin layer was thick, arcing and tracking in the resin resulted. This undesirable phenomena appears to be due to the relatively high activity of some resins which leads to breakdowns when subjected to R.F. fields of the necessary field strength. Although the arcing and tracking problem can be reduced significantly if the R.F. field is applied transverse to the glue line, transverse application reduces the efficiency of the process.
Application of microwave energy has been used in recent years to cure, in composite masses, adhesives which have cure rates which are accelerated by the application of heat. Microwave heating can be more rapid, that is, it can provide a shorter cure time, than conventional heating or hot press processes, and therefore allows for a continuous production technique as compared to batch processes. Arcing and tracking common with the R.F. technique are also not a problem. For example, U.S. Pat. Nos. 4,018,642 and 4,020,311 disclose techniques for simultaneously applying microwaves and pressure to curable assemblies.
An improved microwave applicator for continuous presses is disclosed in U.S. Pat. No. 4,456,498 ('498), and the present invention is an improvement thereon. (The '498 patent was also cited in recent U.S. Pat. Nos. 4,609,417, 4,879,444 and 4,906,309.) The '498 patent shows a pair of endless belts forming a nip region, a press chamber defined by the belts in the nip region and by two side walls, a means for applying microwaves to the curable assemblies through a waveguide which forms an interface with the press chamber located in an opening in the side wall, and a window or dam at the interface between the waveguide and the press chamber and having sufficient strength to withstand the lateral pressures exerted thereon by the curable assemblies as they are being pressed and to thereby block the entry of the assemblies into the waveguide. This window was constructed of a material which is strong, rigid, abrasion resistant, impermeable to adhesives and transparent to microwave energy. Ceramic materials are examples of such materials, and a preferred ceramic material is aluminum oxide (alumina). The applicator system as disclosed in the '498 patent works well on product depths of four inches or less.
An example of a more recent applicator is shown in FIG. 1 generally at 100. (This applicator is prior art for U.S. patent practice, since it has been in secret commercial use for more than one year.) Referring thereto, it is seen that the applicator waveguide 102 is shaped with a fifteen degree angle as shown by reference numeral 104 in a rapidly expanding horn to define an opening 106 having a depth or height of 7.6 inches as shown by dimension 108. The location of the quarter wave trap of this waveguide is shown at 109. The inlet to the horn or waveguide 102 as shown by dimension 110 is 2.17 inches. Positioned behind the ceramic window 112 is a piece of Teflon 114 which is about two inches thick and 9.75 inches wide. The front face of the ceramic window 112 is ten inches wide and has a rectangular configuration. This design for a 7.6 inch depth product worked relatively well and did not generally present any tremendous heating pattern problems. Some degree of uneven heating and occasional product browning was experienced with the 7.5 inch depth product using the applicator or waveguide 102 of FIG. 1, but when the size of this applicator was increased for an 11.4 inches opening, to produce a desirably larger product, the uneven heating patterns worsened and became unacceptable.
FIG. 2 shows generally at 120 a temperature profile using a rapidly expanding horn similar to that of FIG. 1 and for a product depth or window opening of 11.4 inches. This is a temperature profile for dielectric conditions of epsilon prime equaling three and epsilon double prime equaling 0.3. The temperature profile in the product 122 shows the extremes of upwards of 170.degree. C. at the edge of the ceramic window 112 and window 112a (of a similar microwave system on the other belt side) and a low temperature of 67.degree. in the center. The profile thus comprises one massive low in the center of the product and two significant highs on the edges, with significant distances between them. Since the product 122 is fourteen inches wide, there is approximately seven inches from one edge or hot center to the middle or colder center. This means for the heating temperature in the microwave product 122 to even out that there must be a steam transport of approximately seven inches from the higher temperature to the lower temperature, and this is too great a transport distance. In other words, in the hot areas of the compressed mat or product 122 a considerable amount of the steam is generated due to the boiling of the water in the mat. Since boiling is an expansion process, a large volume of gas is created from the small volume of liquid which then tends to flow under pressure gradients to smooth out the low and high temperature spots. A significant evening of the high and low temperatures can only take place, however, if the highs and lows are not spaced too far apart, which is not the case with the "prior art" of FIG. 2. In fact, if the product of FIG. 2 were allowed to sit for ten or fifteen minutes, a chain saw cut then made through it and a thermograph (not shown) taken of the resulting cross-section, the temperatures in the cross-section would vary from thirty to forty degrees.
The main effect of an inconsistent temperature profile 120 is the resulting inconsistent normalization of the product 122. If the product has been heated to about 100.degree., for example, and then wetted, it will later spring back to the remembered prior size. On the other hand, if it is heated above 120.degree., not only has the glue been cured but the lignin has also softened and actually melted and the wood fibers caused to slide internally.
Similar problems have been experienced in other work and corrected using a steam post treatment of wafer board. Some times wafer board is hot pressed too quickly in order to speed the press cycle and the center of the board does not reach a sufficient temperature but rather only a point where it cures enough to hold itself together. Consequently, if the wafer board is later wetted it can spring up to approximately double its thickness, which is unacceptable for most end uses.
The present composite wood product (122) as described, for example, in copending U.S. application Ser. No. 07/555,000 ('000), filed Jul. 23, 1990, and entitled "System for Oriented Strand Layup," (and in Canadian application Serial No. 2,022,900-4), if cured correctly, has only about a two or three percent retained spring back in the compression direction after wetting and drying and in the other directions behaves similar to natural wood. In other words, if the present product is wetted and dried it will return to its original dimensions except in the compression dimension where after drying it will be two to three percent thicker than it originally was. If the higher temperatures are not obtained consistently, then a five or even ten percent increase in thickness after drying is experienced.
This swelling is undesirable for nearly every application since wood that is dimensionally stable is easier to engineer and to service and better able to survive a change in the elements. Often during construction, wood becomes quite wet and only by the fact that it has been placed inside a building that is eventually enclosed does natural evaporation dry the wood to an equilibrium of from about six to sixteen percent.
The (wood) product 122 resulting from the temperature profile 120 of FIG. 2 would most likely be a non-functioning beam. A temperature of 100.degree. C. is needed to cure the glue. It is unlikely that the 120.degree. area and the 140.degree. area would have enough energy to bring the 67.degree. center area to a full 100.degree. temperature before the outside, currently at 170.degree., overheats to the point of browning the wood, which destroys the lignin cellulose matrix. In other words, under cured centers and edge browning result from the FIG. 1 applicator 100 when adapted and used on thicker products. A more even energy flow through this thick product is accordingly needed.
As previously mentioned, a microwave transparent dam 112 has been secured in a microwave curing system 100, such as that of FIG. 1 or of the '498 patent, to the outlet end of the waveguide to prevent the mat, as it is being conveyed and compressed thereagainst and therepast, from entering the waveguide. The dam thus must be strong enough to resist pressures of many hundreds of pounds per square inch. As the dam or window is made larger, for example ten inches across and 11.4 inches in depth to accommodate the larger or thicker product, thermal cracking thereof often occurs.
Another problem in the past has been that the final structural wood products from microwave curing presses of ten have uneven density profiles. If the temperature or moisture contents of the incoming mats are not consistent within a few degrees, instabilities in the change of temperature development, that is uneven heating occur in the microwave press for two reasons. First, the dielectric constant epsilon, both its real and imaginary parts, increases as temperature increases, which means that more energy is deposited into areas that are already warmer. This has a multiplier effect; that is, the warmer these areas get, the move they attract energy, and so forth. A second factor is that these layups are comprised of wood fiber, and wood is compressed as it is microwave heated. The wood is softer when it is warmer, and the warmer part is more easily compressed in the microwave field. As it compresses, the warmer areas take up more of the compression, soften and compress to a higher density sooner, which again increases their dielectric absorption. That is, more energy focuses into those areas, and as this happens they become softer and compress more readily and instability again results. Both of these factors work against even heating and even final product density in wood composite products.
For example, in a wood product with an 11.4 inch by 14.75 inch cross-section, the top and the bottom of the mat can be about 25.degree. to 30.degree. C., while the center two-thirds of the mat can be about 35.degree. to 50.degree. C., due to the natural progression of the water uptake. The center heats up since the water chemically binding to the cellulose lignin structure in the mat is an exothermic activity. As the mat progresses through the press, the top and bottom of the mat can have specific gravities of 0.5 gram per cubic centimeter, while that of the center two-thirds can be 0.6 to 0.65 gram per cc. Further, the moisture content of the 0.5 gram per cc top and bottom areas is about 12 to 13% of the dry basis of wood, while the 0.6 to 0.65 gram per cc has about 9 to 10% moisture content. The density gradient is important to these parameters, since whenever there is a density gradient there is also a strength parameter gradient. Accordingly, there is a different thermal normalizing effect on the compressed mat and thus a different moisture response. The cooler areas tend to expand more rapidly, more readily and more permanently on wetting, which can lead to bowing or splaying of the final product. The resulting density gradient thus has been found to be due to two factors. One is the uneven temperature and moisture profile of the mat as it enters the microwave press, and the other is the uneven microwave deposition pattern of the microwave applicator(s).
A prior art attempt to remedy this density gradient problem has been to raise the temperature of the entire mat. The temperature was raised by insulating the top and the bottom of the mat and then providing an oil heating system around the conveyor itself. More particularly, oil heating lines were positioned along the sides of the trough and heating devices underneath the bottom and insulation covers placed on top of the mat as soon as the last strands were deposited. The mat was thereby lifted out of the very sensitive operating range where these control parameters have their biggest effect. A mat that is entering the press with a 50.degree. to 60.degree. C. temperature has already experienced the bulk of its softening. Thus, even though it still has a temperature gradient of five to ten degrees, this gradient has less of an effect on the final product.
When the temperature of the entire mat is raised sufficiently, the mat behaves reasonably consistently in the microwave heating process. It does not compensate for inadequacies in the evenness of the microwave heating, however. The prior art system of heating the entire layup to make it hotter was thus not an attempt to control either the moisture or temperature beyond an even mat nor did it achieve an even mat temperature. A benefit of raising the temperature of the entire mat is that the effects of the ambient temperature are reduced but not eliminated. In other words, on hot summer days there is a different mat self-heating profile than on cold winter days, and these effects are reduced to a certain extent by heating the entire mat.
A significant disadvantage of this "whole mat" heating technique, however, is that if the temperature gets too high, for example to 60.degree. or 70.degree. C., then the glue in the mat can be precured, making the product useless. The product may still look good, consolidated and strong but if the glue was even partially cured before the final compression and microwave heating, there is little left to hold the wood strands or composite assemblies together. Aside from the precuring problem, there is also the problem that the entire mat as a practical matter is difficult to heat evenly since there are differential chemical reactions occurring with this water uptake. The mat is simply not stable enough to be totally heated to an even, higher temperature.
A prior art technology in the board industry for reducing press curing times is to radio frequency (RF) preheat the mat before it reaches the press. That is, an RF field is applied to the uncompressed mat to raise the temperature thereof to 50.degree. to 70.degree. C. or even higher before final compression and heating with a hot press. This board forming technique is usually a batch and not a continuous process, however, and the purpose of the RF preheating is to shorten the pressing time in the hot press. This mat is also formed very thin so that there is no significant steam transport within it. Further, this prior art board forming process does not involve any significant or positive compression of the mat. Thus, any small irregularities in preheating will not magnify during the process. When a mat is to be simultaneously heated and compressed (such as in the present processes described in detail below) compressibility during heating is very important and any instabilities tend to magnify.